This invention relates to transparent adhesives containing isocyanate groups and to their use for the production of composite films.
The production of composite films for the packaging industry is assuming increasing proportions because composite films can be built to have made-to-measure properties by combining various films. Polyurethanes are the chosen adhesives because they guarantee firm adhesion of the films to one another, are odorless and, by virtue of their transparency, and do not adversely affect the appearance of the goods to be packed. Whereas, in the past, the majority of adhesives were applied from solution, the trend is now towards low-solvent systems (cf. DE-A-3,431,688) and solventless systems (cf. for example DE-A 2,549,227 and 3,815,242). The weak points of these new systems are, in particular, the relatively poor initial adhesion which can cause problems, especially in the high-speed processing of composites. In addition, composites in which aluminum is one of the film materials to be bonded show poor strength values, particularly when they are subsequently subjected to sterilization with steam.
Numerous possibilities for improving the adhesion of aluminum composites have already been proposed (cf. EPA-349,838, DE-A-2,857,281 and 3,529,176), all being distinguished by the fact that additives, such as for example silanes, epoxides, phosphoric acid and/or anhydrides, are added.
Polyurethanes containing urea groups for adhesive applications are not new, but hitherto, they could only be produced in solution by reaction of an isocyanate prepolymer with an excess of a polyamine (U.S. Pat. No. 3,931,116) or bis-ketimine (U.S Pat. No. 4,137,276).
On the account of the instability of ester bonds in the presence of amino groups, storable products can only be obtained by this process if polyethers are used. On account of their high reactivity, the products containing amino groups can only be further reacted with polyepoxides and not with polyisocyanates.
Polyurethane urea solutions which are stable in the absence of moisture can be obtained by reaction of a polyisocyanate prepolymer with aminosilanes to form terminal trialkoxysilane groups (DE-A-2,155,258) which rapidly crosslink, under the effect of water.
Apart from the large amounts of solvent required for stabilization, it is the sensitivity to water which is an obstacle to practical application of these polyurethane urea solutions.
DE-A 3,815,237 relates to a process for the production of low-monomer polyisocyanates by selective reduction of the modified polyisocyanate content by addition of water wherein the products obtained are used for coating purposes.
On account of the basically already low monomer contents of the polyisocyanates, the reaction with water gives polyisocyanate urea products having a very low content of urea segments from which no adhesion-promoting effect can be expected.
The problem addressed by the present invention was to provide adhesives, more particularly, adhesives for composite films having improved strength and, in particular, initial strength.